Structure and phase transition of synthetic NaNa2Mg5Si8O22 (OH)(3)

Na Na2 Mg5 Si8 O22 (OH)3 is the first amphibole composition with a rigorously documented excess of protons. It is triclinic, space group C-1, and the unit-cell derives from a tripling of the b edge. This symmetry allows off-centring of the B-cations [1], as well as different conformations of the two sides of the double chains. The correlated displacements of BNa atoms permit the incorporation of excess protons in pseudo-tetrahedral cavities between two adjacent chains of tetrahedra belonging to different I-beams. Bond-valence calculations and crystal-chemical analysis suggest that the excess proton is bonded to the oxygen atom at the O4 site, and is hydrogen bonded to that at the adjacent O2 site. The observed cation arrangement is coherent with previous 29Si- and 1H MAS NMR studies of this amphibole [2]. Na Na2 Mg5 Si8 O22 (OH)3 undergoes a triclinic-tomonoclinic C-1 ® C2/m phase transition in the T range 100-160 °C [2]. The transition has now been studied by FTIR in the T range 25-250 °C. In the OH region, the room-T spectrum shows a triplet of very sharp bands, at 3740, 3727 and 3711 cm-1, respectively, which can now be assigned to three distinct O-H dipoles locally interacting with a strongly ordered A-cation, and a very broad band centered around 3300 cm-1. There is a systematic peak shift and intensity decrease with T. At 180 °C the three peaks merge into one single absorption at 3723 cm-1, and no significant change is observed beyond this T. Part of the broad band centered at 3300 cm-1 vanishes with increasing T, while a strongly asymmetric and well defined peak at 3430 cm-1 is resolved. The intensity of this peak remains constant at any T, even for long heating stages at T > 250 °C. This absorption is assigned to the excess proton, and the measured frequency confirms that it is strongly involved in hydrogen bonding with surrounding oxygen atoms; the width of the band implies local disorder. All these changes are completely reversible during cooling. [1] Cámara et al. Am. Mineral, 2004, in press. [2] Liu et al. Eur. J. Mineral., 1996, 8, 223.